The delivery of products of consistent dimensions, properties and capabilities, etc., necessitates quality control methodology. No special problems exist in obtaining homogenous and uniform products on a batch or continuous production basis for certain products (e.g., wrought products). Such products allow proper statistical sampling and testing whereby acceptable representative product quality information may be derived.
Other products vary greatly in quality precluding random or scheduled sampling from a number of these products. Under these conditions even apparently parallel production conditions will not give insurances that a selected product will meet the necessary specifications for its intended service. Testing of each product is therefore generally the only method of predicting that the selected product or sample is qualified for its intended use. Obviously, however, such testing is only viable if the sample is not damaged or destroyed by the testing.
For example, castings made of metals such as aluminum, present serious quality control problems since the castings produced within a production run of the same or a different manufacturer vary substantially in physical properties. Tensile properties, e.g., may vary between casting zones in the same casting due to, e.g., differences in local rates of solidification. Natural variations may also occur between castings due to slight changes within the acceptable limits for adding of the constituents to form the alloy and the latitudes of time and temperature of heat treatment processing.
The present state of the casting art, especially the aluminum casting art, is not capable of economically mass producing castings of statistically controllable or predictable properties. Furthermore, the tests from which a skilled artesan can usually select for determining mechanical properties such as percent elongation or ductility, yield strength, tensile strength, weaken or destroy the casting.
Recently, mechanical properties of castings have been ascertained by nondestructive procedures wherein test bars poured from the same metal and at the same time as the actual casting are destructively tested and by means of coupon test bars which are poured as an integral part of the casting and broken off at the time of completion of any heat treatment of the cast article. In the latter procedure the coupon test bars are also destructively tested. However, both the former and the latter procedure, exemplified by U.S. Pat. No. 3,496,766 issued Feb. 24, 1970 to Stein, provide a mere differential determination of the structural properties of the casting. Both methods are therefore incapable of providing reliable information on the properties of the metal in a particular zone which may have responded to heat treatment differently from another zone.
Other conventional non-destructive methods have attempted to use microstructural parameters such as dendrite arm spacing, dendrite cell size and dendrite cell interval to estimate the strength and the ductility of metals. However, such testing methods are difficult to use and produce unreliable results. These secondary arm linear parameters seem to be most indicative of solidification rates and grain refinement. However, these dendrite measurements are extremely difficult to make due to the improbability of obtaining the exact metallurgical mount surface orientation needed to view their true dimensions.